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When datum features are modified at MMB or LMB, the datum feature and the features being controlled may be able to shift relative to the datum axis or centerplane. On the drawing shown below, datum feature B, in the position callout of the four hole pattern, is modified at MMB. This means the datum feature simulator for datum feature B would have a theoretical design size of Ø19.6.
Since the actual datum feature could be produced as large as 20.2, the datum feature could shift as much as 0.6 total. This means the four-hole pattern may shift out of position in one direction while the datum feature shifts in the other direction as shown. A very common error made at inspection is to use this allowed datum shift as a bonus on the features being controlled. This would allow the features to be out of position to each other more as the datum feature departs from MMB. This practice does not agree with the drawing or a hard gage, should one be produced.
This tip is in accordance with ASME Y14.5-2009 and ASME Y14.5-2018.
This tip was originally released in November 1998.